This invention relates to a brake-slip-controlled brake system with a pedal-operated master cylinder assisted by auxiliary energy, with sensors for detecting the rotational behaviour of the wheels and the vehicular velocity or respective measurable values. Valves are provided in the pressure medium supply lines from the master cylinder to the wheel brakes. An electronic circuit configuration is provided for logically combining and handling the sensor signals as well as for generating valve control signals. Valves are provided for feeding pressure medium from the booster chamber into the working chamber of the master cylinder.
A brake system of this type is known (German patent application No. P 30 40 562.0) where a master cylinder is provided which has two master cylinder pistons which are connected one after the other, and which is operable by way of a piston rod connected with a brake pedal. When an actuating force is applied to the brake pedal the gaskets of the master cylinder pistons override so-called expansion ports and thus interrupt the connections between the working chambers of the master cylinder and an unpressurized storage reservoir. Upon a further displacement of the master cylinder pistons there results a pressurization of the working chambers of the master cylinder and the vehicular brakes connected to the working chambers are applied. Associated with the vehicle brakes are electromagnetically operable valves which keep the wheel slip down to a favorable extent upon suitable control by slip monitoring electronics.
Between the pedal-operable piston rod and the master cylinder pistons a throttle valve is arranged which consists of a piston axially displaceable within the master cylinder bore and having a pedal-close valve seat and of a valve closure member formed fast with the piston rod. As long as there is no danger of a lock-up the pedal force will be transmitted mechanically to the master cylinder pistons by way of the piston rod and the piston carrying the valve seat of the throttle valve. When an unduly high slip at one or several of the vehicle brakes is detected by the slip monitoring electronics, an electric-motor driven pressure medium pump will be operated. The pump will suck hydraulic medium from the unpressurized storage reservoir, supplying it to the throttle valve. Thus, as a function of the actuating force applied to the brake pedal a dynamic pressure will result at the delivery side of the pressure medium pump. The pressure will be tranmitted to the pedal-close front sides of the master cylinder pistons where, by way of axial bores of the master cylinder pistons and by way of gaskets acting a non-return valves, the pressure medium may flow into the brake circuits connected to the working chambers if pressure medium had been tapped from the wheel brakes for the purpose of a pressure reduction.
Moreover, the dynamic pressure generated at the delivery section of the pump by means of the throttle valve will pressurize a positioning piston having a larger effective surface than the master cylinder pistons and being displaced contrary to the direction of actuation against a stop formed fast with the housing. In doing so, the positioning piston will come into mechanical contact with the piston carrying the valve seat of the throttle valve and will return the latter, if necessary, against the pedal force in the brake release direction. The positioning piston will ensure that there will always remain a sufficient amount of fluid in the working chambers of the tandem master cylinder. A drop of the master cylinder pistons down to the bottom of the master cylinder will thus be impossible.
Upon a failure of the pressure medium pump or of the pump drive there will be no pressurization of the positioning piston. Upon an application of the brake in such cases of trouble, the piston with the valve seat will abut on the positioning piston and displace the same against the force of a relatively weak compression spring in the direction of actuation. This displacement will be transmitted to the master cylinder pistons as in normal braking actions, thus respectively the working chambers of the master cylinder and the wheel brakes connected to them being pressurized. A normal operation of the motor vehicle's brakes will therefore be ensured in any case in the above-mentioned circumstances of trouble.
In brake systems of the type referred to above it is known that at first a certain servo pressure must be generated by the brake valve or rather control valve before pressurizing the wheel brakes connected to the master brake cylinder.
It is thus an object of the present invention to further develop a hydraulic brake system with the characteristics described above such as to ensure that, during the braking action, a drop of the brake pedal will be avoided and the pistons of the master cylinder will remain in the positions adopted by them immediately before the dynamic pressure medium supply into the wheel brakes. Moreover, it is to be ensured that upon release of the brake pedal the same will return at once into its initial position.